1. Technical Field
The present invention relates to a mounting substrate, and to an electronic device in which this substrate is used.
2. Description of Related Art
Light emitting devices that make use of light emitting elements have been put to use in a variety of light sources in recent years. Various methods have been employed to raise the light emission output and achieve higher luminance in these light emitting devices. For instance, a light emitting device in which a light emitting element is mounted on a ceramic package with high thermal conductivity has been used to supply a large amount of current and obtain a high-output light emitting device.
Meanwhile, with a such light emitting device such, in the case where a wiring layer, a pad, or the like is provided on the surface by using solder or another such joining member, there is the concern that cracks will be generated in the joining member due to thermal stress produced by the difference in the coefficient of linear expansion between the ceramic and the mounting substrate.
In view of this, there is one approach in which the joint is strengthened by employing a so-called non-solder mask defined (NSMD) structure, rather than a solder mask defined (SMD) structure, as the pad structure for the wiring layer of the mounting substrate, and allowing the joining member to work its way into the lateral surfaces of the pad as well. Also it is suggested that a structure combined SMD structure and NSMD structure (for example, JP2009-111279A).
When an NSMD structure is employed, however, the wire width of the wiring layer linked to the pad inevitably ends up being partially narrowed, and there is a risk that thermal stress will cause breakage at this narrowed part. Also, with an NSMD structure, the surface area of the wiring layer and the pad is smaller than with an SMD structure, so there is also a concern that heat dissipation will decrease.